This invention relates to method and apparatus for forming very uniform coatings on discrete microsized particles and more particularly to a method and apparatus for applying such coatings on discrete microsized particles by electroless techniques.
A mixture of deuterium (D) and tritium (T) is a preferred fuel for laser fusion, primarily because the least energy is required to cause these two isotopes to undergo thermonuclear reaction. It is desirable that the DT mixture acted upon by the laser radiation be as dense as possible. The optimum density is achieved by cooling the mixture sufficiently that it becomes a solid. This, however, requires temperatures below 20 K which imposes very severe constraints not only on the manufacture, but also on the handling of laser fusion targets.
Alternatively, hollow, spherical, DT-gas-filled targets with diameters ranging from 30 to greater than 200 .mu.m and with contained fuel pressures varying from 10 to 1000 atm (at 298 K) are of interest for laser fusion. The primary gas-containment vessels of these targets are hollow microspheres called microcapsules. The targets are filled by diffusing DT fuel gas through the walls at elevated temperatures, taking advantage of the exponential temperature dependence of the permeability to allow the gas to be retained for useful times at room temperature. Thus, when the microcapsules are placed in a deuterium and tritium gas mixture a desired ratio at high pressure and elevated temperature, the deuterium and tritium readily enter the microcapsules and equilibrate to the surrounding gas pressure. When the microcapsules are cooled to room temperature, the diffusion rate through their walls is greatly reduced, so that the DT mixture within the microcapsules remains at high pressure for times which permit useful storage before the targets are irradiated by the laser.
To most effectively produce the compression of the DT fuel necessary for thermonuclear reaction, it is desirable that the fuel be surrounded by a pusher shell of high-Z metal. Presently available microcapsules are not composed of materials having the desired high-Z characteristics, so that it is necessary to coat them with a layer of high-Z metal. This metal shell of the laser fusion target must be fully dense and of very uniform thickness, with aspect ratios in the range of from 10/1 to 100/1. It should have a minimum tensile strength of 690 MPa.
It is well known in the art that electroless techniques are well suited to the deposition or plating of certain metals on various surfaces. Unfortunately, conventional electroless plating apparatus and methods have been found to be largely unsuccessful in producing metal coatings of the requisite thickness and uniformity requirements on microcapsules. A primary problem has been the tendency of the microcapsules to agglomerate and/or bridge during the coating process. By bridging is meant the joinder of the coatings of two or more microcapsules to form one common structure.